E. -W. Saw | Searcharxiv

arXiv2026

A simple experiment for observing clustering and dynamics of coalescing particles in air turbulence

A novel experimental platform is developed to investigate the dynamics of inertial particles (micro-droplets) in air turbulence. The goal is to observe particle collision and coalescence in turbulent flows, focusing on its impact on the radial distribution function (RDF) and relative velocity statistics. The main tool is a three-dimensional Lagrangian particle tracking (LPT) system, designed for high-resolution measurements at sub-Kolmogorov scales. The system uses LED illumination with high-speed spinning-disk atomizers, enabling tracking of particles of approximately 10~$μ$m and larger under controlled turbulence. A minimum resolvable particle separation of $r/η\approx 0.1$ is achieved. A central contribution is the identification and mitigation of three dominant sources of spurious particles: FMIS, IIS, and TIF. An angle-based geometric filtering criterion strongly suppresses FMIS artifacts on RDF. These procedures establish a validated workflow for reliable small-scale statistics. Using this framework, RDF and a normalized pseudo-collision rate are measured at near-contact separations for particles with Stokes numbers $St \approx 0.2$--$1.0$. Sub-Kolmogorov clustering increases with Stokes number, and near-contact statistics are consistent through the filtering strategy. This study extends LPT limits and provides a reliable methodology for investigating inertial-particle dynamics at previously inaccessible spatial scales.

physics.flu-dyn↗

arXiv2021

On the investigation of properties of superfluid $^4$He turbulence using a hot-wire signal

We report hot-wire measurements performed in two very different, co- and counter-rotating flows, in normal and superfluid helium at 1.6 K, 2 K, and 2.3 K. As recently reported, the power spectrum of the hot-wire signal in superfluid flows exhibits a significant bump at high frequency (Diribarne et al. [1]). We confirm that the bump frequency does not depend significantly on the temperature and further extend the previous analysis of the velocity dependence of the bump, over more than one decade of velocity. The main result is that the bump frequency depends on the turbulence intensity of the flow, and that using the turbulent Reynolds number rather than the velocity as a control parameter collapses results from both co- and counter-rotating flows. The vortex shedding model previously proposed, in its current form, does not account for this observation. This suggests that the physical origin of the bump is related to the small scale turbulence properties of the flow. We finally propose some qualitative physical mechanism by which the smallest structures of the flow, at intervortex distance, could affect the heat flux of the hot-wire.

physics.flu-dyn↗